The invention relates to an amplifier circuit with offset compensation, in particular for magnetic field sensors operated in chopped mode.
Sensors, for example magnetic field sensors, usually need an offset error correction in order to operate precisely. In the case of integrated Hall elements operating as magnetic field sensors, an offset error can arise, for example, because of fabrication-induced bending of the silicon platelet (mechanical stress) or as a result of lithographic inaccuracies during the production process. Further offset errors can be caused by the external wiring of the Hall sensor, for example as a result of offset voltages in amplifiers connected downstream of the Hall sensor. U.S. Pat. No. 5,621,319 to Bilotti et al. specifies a method for compensating for the offset voltage normally occurring in Hall sensors. There, two pairs of terminals that are arranged orthogonally with respect to each other are provided on the Hall sensor. In each case one pair of terminals is fed with the excitation current and the Hall voltage is available on the other pair of terminals. A change between the pairs of terminals is made at the chopping cycle rate. In this way, the alternating voltage error signal is reduced.
A development of that chopped-Hall principle to a Hall sensor having N pairs of connecting terminals, which are interchanged cyclicallyxe2x80x94it is thereby possible for the Hall voltage to be picked off on one pair of terminals in each case which is arranged orthogonally with respect to the pair of terminals of the excitation currentxe2x80x94is specified in the document xe2x80x9cSpinning-Current Method for Offset Reduction in Silicon Hall Platesxe2x80x9d by Peter Jahn Adriaan Munter, Delft University Press, 1992, page 12.
The common factor in the above-described offset compensation methods for magnetic field sensors is that in the amplifier connected downstream of the magnetic field sensor, both the useful signal and the chopped error signal have to be amplified considerably for further processing or demodulation. This requires high driveability of the amplifier stages, which stands in the way of the requirement for a lower and lower internal operating voltage. In order to suppress the alternating voltage error signal adequately, comprehensive circuit measures are required.
Furthermore, the above-described sampling of the useful signal then leads to an additional error when the useful signal changes. In the case of time-critical applications, this error signal additionally has to be filtered out of the useful signal.
Further disadvantages of the above-described offset compensation method reside firstly in the fact that, because of the offset-afflicted buffer amplifiers which are used in the sample-and-hold circuits, additional, undesired offsets are produced and, secondly, further offsets arise as the result of the use of summing resistors which have mismatches. Although these offsets are reduced in accordance with the chopped-Hall principle, they are not eliminated completely.
It is accordingly an object of the invention to provide an amplifier circuit with offset compensation which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which is suitable for Hall sensors operated in chopped mode, requires a low drive range and needs little expenditure on filtering in order to suppress alternating voltage error signals.
With the foregoing and other objects in view there is provided, in accordance with the invention, an amplifier circuit with offset compensation, comprising:
a signal source for outputting a first measured signal in a first clock phase and a second measured signal in a second clock phase, each of the first and second measured signals having a useful component and, possibly, an offset component;
an amplifier having an input side connected to said signal source for receiving the measured signals and having an output side for carrying amplified measured signals;
an error signal demodulator connected to one of said input side and said output side of said amplifier for receiving the measured signals; and
said error signal demodulator having an output connected to an input at said input side of said amplifier for supplying the offset component in antiphase.
In other words, the objects of the invention are achieved by an amplifier circuit with offset compensation as outlined. The amplifier circuit includes a signal source, on which a first measured signal can be tapped off in a first clock phase and a second measured signal can be tapped off in a second clock phase, the measured signals in each case having a useful component and possibly having an offset component. The circuit furthermore has an amplifier which, on the input side, is connected to the signal source for the supply of the measured signals and on whose output the amplified measured signals are provided. An error signal demodulator is connected to the input or output of the amplifier in order to supply the measured signals, and the demodulator has an output that is connected to an input of the amplifier in antiphase in order to supply the offset component.
According to the principle of the present invention, the offset component of the first and second measured signals is in each case coupled out of the measured signals by an error signal demodulator, the error signal is demodulated and then the demodulated error signal is fed back to the input of the amplifier. In this way, the error signal or the offset component of the measured signals is already eliminated at the amplifier input, so that the drive range of the amplifier only has to be matched to the useful component of the measured signals. Consequently, the drive range of the amplifier can be reduced considerably. The result is, therefore, a saving of chip area with higher accuracy of the circuit. In addition, the result overall is a lower delay time, caused only by the amplifier circuit. Furthermore, error signals which are caused by changing the useful signal are avoided as a result of avoiding sampling processing in the main signal branch.
After the amplifier circuit has been switched on, first of all both the useful component and the offset component of the measured signals and also amplifier offset signals are amplified in the amplifier. Even after a few clock cycles, the error signal antiphase coupling leads to the offset components being considerably reduced without falsifying the useful components.
In the antiphase coupling of the error signal, an integrator can be provided, by which means offsets are virtually completely eliminated.
Further advantages consist in the lower expenditure on filtering of the measuring signals, since the output ripple in measured signal ramps is reduced sharply. Since, for the further processing of the measured signals to be carried out on the output side of the amplifier, in particular of the useful component of the measured signals, a lower analog bandwidth is required, the noise characteristics of the amplifier circuit are improved. Finally, the aliasing errors are reduced in the amplifier circuit described, since no sampling-signal processing is carried out in the main signal path.
The signal source can be a magnetic field sensor.
In a preferred embodiment of the present invention, the measured signals are carried by using the differential path technique. In this way, common mode interference is suppressed.
In a further, advantageous embodiment of the present invention, a modulator which has means for reversing the polarity of the measured signal lines is arranged between magnetic field sensor and amplifier, and a demodulator which has means of reversing the polarity of the measured signal lines is arranged at the amplifier output. The means of reversing the polarity of the measured signal lines in the modulator and demodulator can be driven at a common chopping cycle rate.
If the signal source is a Hall sensor operated in chopped mode, the amplifier circuit can have 90xc2x0 changeover switches for excitation current connections and signal outputs for the measured signals, which can be arranged in the modulator.
In accordance with a further, advantageous embodiment of the present invention, the error signal demodulator has a first low-pass filter and an amplifier connected downstream with a resistance on the output side. The amplifier can be a transconductance amplifier (OTA, operational transconductance amplifier). Instead of the low-pass filter, a summing junction or an integrator can be provided, in each case with an amplifier connected downstream.
In accordance with another feature of the invention, the amplifier can have a second low-pass filter connected downstream, which can be implemented with RC elements constructed in analog fashion. This makes it possible, in accordance with the principle of dynamic element interchange, to interchange the tolerance-afflicted resistors periodically, in order to compensate for their fabrication-induced tolerances.
The magnetic field sensor can have a Hall element. If the signal source is not a Hall element, the measured signals do not necessarily have to have an offset component. In this case, nevertheless, compensation is carried out, specifically the compensation of the offset of the (chopper) amplifier itself.
For chopping operation, the Hall element can have two orthogonal pairs of terminals, wherein in each case the excitation current is supplied to one pair of terminals and the Hall voltage, that is to say the first or second measured signal, can be tapped off at the other pair of terminals.
The error signal can be coupled out on the input side or output side of the amplifier.
The first low-pass filter in the error signal demodulator can of course be replaced by summing the signal voltages over the two clock phases or by averaging the voltages over the two clock phases. The first low-pass filter in the error signal demodulator can be replaced by an integrator.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a amplifier circuit with offset compensation, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.